1. Area of the Art
The present application relates to systems for the handling and local transport of disc shaped flat sheets of stiff but flexible materials such as integrated circuit wafers and the like materials. Particularly, the present invention relates to an intelligent integrated circuit wafer location and handling system and a method for selectively positioning and processing both sides of a subject wafer.
2. Description of the Prior Art
Semiconductor wafers are produced by complex multi-step processes. Sophisticated integrated circuit type electronic chips are derived from wafers during processes involving often greater than 100 steps. Many of these steps require extremely accurate positioning of the chips because the submicron range technologies used in manufacturing the chips are both error and inspection intensive. Also, the wafers before processing are extremely expensive, and become even more valuable after processing. There is therefore a strong need for heightened control of processing and handling methods for the involved wafers.
It follows that the high production rates required for production of integrated circuits necessitate that the wafers upon which individual circuits are located be processed rapidly and in batches. Typical wafers being processed have diameters ranging from about 4 inches to about 12 inches. Such wafers are generally housed for processing in cassettes, or caddies in closely stacked vertical arrangements.
Processing generally entails separate removal of each subject wafer from its housing cassette and loading of the same into the processing equipment utilized, followed by return of the processed wafer to a cassette or carrier. The receiving cassette may be different than the first cassette, and the fragile nature of the wafers, generally silicon, provides further constraints. Removal, processing and repositioning of these varying sized wafers has created a longstanding need for more efficient apparatus and methods for processing them. Further, because the wafers are thin as well as formed from brittle materials, the pressure exerted on the wafers by the handling device can be critical and the gripping pressure must be carefully controlled to minimize bending, cracking or chipping of the costly wafers while still assuring a firm grip on the wafer to avoid dropping it during handling.
Patented Apr. 14, 1992, the WAFER INSPECTION SYSTEM of U.S. Pat. No. 5,105,147 ("Karasikov" et al.) is typical of the state of at least one aspect of the existing art. The disclosed system is for the semiautomatic inspection of printed circuits on silicon wafers. Included in the Karasikov patent are the combination of a floating table, and a robotic arm optical inspection device which includes a sophisticated optoscanner for the alignment and positioning of a wafer.
Karasikov removes involved wafers by applying a vacuum to a narrow zone at the circumference of a wafer. The mechanism of the Karasikov patent highlights problems which result in many of the processing errors ameliorated by the teachings of the present invention.
Likewise, U.S. Pat. No. 5,504,345 ("Bartunek" et al.) which issued Apr. 2, 1996, disclosed a dual beam sensor and edge detection system and method. Two light sources, or solid state lasers, are used to detect the edges of the involved wafers. The Bartunek patent essentially shows that lasers may be used for the detection of, for example, the reflective surface of a wafer or optical disc.
Other known systems for wafer handling similarly either address improvements in locating wafers or quasi-automated means for handling wafers. It would be highly advantageous to have the capability for concurrently improving the performance of both of these functions within a single system. Capitalizing upon the use of lasers without the drawbacks of vacuum-based technology would solve many longstanding needs.
By way of example, current technology often uses a vacuum chuck mounted on a robotic arm to remove or replace individual wafers in the cassettes. Since the position of each cassette and each wafer within the cassettes is unique, the location of each disk within the three ordinal planes ("X, Y, and Z") relative to some reference has to be entered into the software driving system controlling the robotics that handle the wafers.
Existing methodology requires mechanical measurement of each location followed by the data being manually entered into the software being utilized. This a time consuming process additionally constrained by the high likelihood of human error. Sufficient differences exist among known cassettes and cassette holders, that a calibration of every cassette to be employed is generally required.
Further, these constraints are complicated by the fact that, for example, in semi-portable processing systems, relocation of any part of the system requires new calibration.
Conventional vacuum chucks further induce harmful artifacts of the processing steps and these artifacts can result in lower industrial efficacy. Any warpage in the employed vacuum chuck may cause malfunctioning because of air leaks. Since the vacuum chucks must be thin and contain air passages, they are difficult and expensive to build. Further, since the wafer is held by the surface, the wafer is prone to slip under the high acceleration rates necessary in high speed processing. Any misalignment of the disk with the arm can cause the system to crash. Contamination of the surface by the arm itself occurs with alarming frequency.
Likewise, a clear need exists for a way to process both sides of a semiconductor wafer. Among the prior art, various attempts at solutions to related problems, and methods for handling wafers for processing are illustrative of the paucity of patents actually addressing the above enumerated constraints. The state of the art clearly shows a need for improvement, such as taught by the present invention.
Another method of lifting wafers is the use of mechanical grippers, U.S. Pat. No. 5,570,920 ("Crisman" et al.) issued Nov. 5, 1996, utilizes a DC motor to drive a robotic finger. Unlike the teachings of the present invention, strain gauges 171, 173, 175 on the inner surfaces of the fingers are used to sense gripping pressure and, once activated, stop the motor.
By way of further example, U.S. Pat. No. 5,435,133, which issued Jun. 13, 1995 ("Yasuhara" et al.) utilizes servo motors which drive robotic fingers based on positioning signals. However, no sensors to indicate or control grasping force were found. Likewise, the complex attaching/detaching portion of the hand portion was the focus of Yasuhara's disclosure, differentiating this patent from the teachings of the present invention.
Additionally, U.S. Pat. No. 5,378,033 ("Guo" et al.) which issued Jan. 3, 1995, utilizes a single drive mechanism for all of the involved mechanical fingers so that they apply a uniform force on the object grasped. The Guo patent teaches a purely mechanical robotic or prosthetic hand. However, the method of controlling the drive mechanism was not apparent, differentiating the Guo patent from the teachings of the present invention.
U.S. Pat. No. 5,280,981 ("Schulz") issued Jan. 25, 1994 uses a load responsive two-speed drive assembly and a slip clutch. See Col 9, line 63-col 10, line 28. Notably, the digit actuation mechanism of the Schulz patent contemplates neither using solenoids nor current generation means wherein a force directly proportional to current is used.
U.S. Pat. No. 5,188,501 ("Tomita" et al.) issued Feb. 23, 1992, was directed to a wafer transfer system which uses a set of claws which pivot under a wafer to serve as a lifting platform for the wafer. The Tomita patent is thus different from the teachings of the present invention because it works by creating a lifting force which cradles the wafer rather than applying a grasping force.
Issued Dec. 22, 1992, "Jacobsen"'s U.S. Pat. No. 5,172,951 does not appear to disclose a tension sensing or controlling technique. This ROBOTIC GRASPING APPARATUS operates with three degree of freedom, yet does not disclose wafer-friendly usages such as those which are an object of the present invention.
U.S. Pat. No. 5,108,140 ("Bartholet") issued April 4, 1992, includes a palm plate and grippers having tactile or other sensors on its upper surface to detect the position of the payload or to provide input to the control mechanism (Col 5. lines 20-37). A parallel vise like grip is generated but no means of detecting or controlling the gripping force appears to be given.
Likewise, "Ulrich"'s U.S. Pat. No. 5,501,49 (issued Mar. 26, 1996) and U.S. Pat. No. 4,957,320 (issued Sep. 18 1990) each use tactile sensors 200, 210 located on the palmar surfaces of the fingers and the palm.
U.S. Pat. No. 4,354,553 ("Rovetta" et al.) issued Sep. 28, 1982 shows a three finger grasping system where the force applied by the fingers is supplied by traction cables 42, 43, 44 along the inner surface of the fingers such that tension applied to the fingers causes the fingers to pivot inward, tightening the grasp on the held object. Sensors 84, 85, 86, shown in FIG. 6 of the Rovetta patent, attached to the tension cables sense the traction force applied thereto, differentiating the Rovetta patent from the teachings of the present invention.
Finally, U.S. Pat. No. 4,653,793 ("Guinot") incorporates strain gauges 26, 28 on the fingers.
Accordingly, since nothing among the prior art has adequately addressed the longstanding needs ameliorated by the present invention, an intelligent integrated circuit wafer handling system is offered to meet these needs.